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Avoiding herbicide resistance

Worried weed scientists gather to discuss the future of weed control without new chemistry

The loss of glyphosate is something that we should all lament,” said Steven Powles, director of Australia’s Herbicide Resistance Initiative. Powles compared the importance of glyphosate to penicillin, calling it a one-in-100 year herbicide.

Powles was speaking to a gathering of weed scientists at the Weed Science Society of America’s second herbicide resistance summit in Washington on September 10.

Many of these seed scientists, including Powles, are deeply concerned about the future of weed control. Mainly because there are no new chemical developments coming down corporate pipelines.

“Don’t hold your breath for a new herbicide mode of action,” Powles said. While we see advertisements for new herbicides, he says, these products are the addition of transgenes to existing herbicides, or mixing 2,4-D and dicamba into new formulations. “Resistance has, can and will occur for these herbicides,” Powles said. “How these are used will be critical to their longevity. They are not silver bullets.”

Powles is concerned about farmers who continue to use practices that contribute to resistance development. “Does there have to be a train wreck before change occurs?”

Part of the problem, Powles believes, is our history of successful weed control. The current generation of farmers have always been able to simply use herbicides to control weeds. “Changing that culture is difficult,” he said. “There’s a very strong belief that new herbicide solutions are imminent.” That perception, Powles said, is fed by industry — companies continue to signal to farmers that new solutions are coming soon.

Other factors also contribute to a “culture” of relying on simple chemical solutions, said Powell. These include:

  • short term decision making;
  • high percentages of rental land;
  • falling grain prices;
  • generic herbicide suppliers;
  • growers used to “easy weed control”; and,
  • growers that do not “fear the weeds.”

Powles described the current situation as HOS: “herbicide only syndrome,” with farmers using just one plan of attack. But, he said, “don’t get depressed. It’s entirely management. It starts by accepting HOS exists.”

Diversity will be the solution, Powles said. “Diversity disrupts resistance evolution.”

Solutions to weed resistance will range from new mixtures of existing herbicides to creative responses like the tow-behind carts that collect weeds behind combines in Australia, or “propelled abrasive grit management” — using air compressors to blast “grit” (a substance made from dried corncob bits or something with similar textures) at weeds growing between rows of corn, shredding the weeds.

Another step Powles advocates is mandatory mode of action labelling, to ensure that farmers have easy access to the information they need to rotate herbicides based on modes of actions, not just brands.

“Industry better get on board,” he said.

Resistance extension

Michael Owen, a professor and extension specialist at Iowa State University agreed that, “historically, there have not been a lot of changes in tactics” for weed management.

Meanwhile, he said, weeds have been farmers’ most consistent problem, although they have don’t get as many headlines as new diseases and novel pests. “The consistency of economic loss attributed to weeds is unprecedented,” Owen said.

Owen said farmers don’t always understand weed management, and the prevalence of resistant weeds. In 2013, he calculated that 77 per cent of Iowa soybean fields had weeds above the canopy in September — weeds that were likely herbicide resistant. However, he said, as farms have expanded, farmers with lots of acres “don’t have time to go and look at these weeds,” or if they do, “the growers don’t recognize them.”

Owen said farmers often don’t identify resistance problems until it’s too late. “Twenty-four to 35 per cent of the weed population must have the resistant trait before growers recognize that they have that problem.”

The best solution, Owen said, is “boots on the ground. Scouting.” This means examining all parts of each field, and doing what it takes to control weeds in each area. In some cases, Owen suggests tillage. Farmers who have adopted non-tillage farming methods are not enthusiastic, but Owen isn’t advocating putting entire farms under cultivation. “Each field is different, and needs to be managed separately,” he said. This might mean tilling only parts of fields, where there are specific problems.

Owen also suggested other changes that could have a big impact in herbicide resistance development. One of these is changing crop rotations — he cited “continuous corn in the upper Midwest” as a factor contributing to resistance. Another is agronomy. For example, changing soybean row spacing can make a difference to weed control. There may be new non-herbicide solutions in the future, such as biological control, robotoics or developments in genomics. However, “are they going to answer all the questions? Absolutely not,” Owen said. “The key is diversity.”

Farmers need to use many different weed control solutions. (Owen referred to this approach as “many small hammers.”) “Grower expectations need to reflect the control that each of these tactics may provide,” he said. Unlike the full control farmers have come to expect from herbicides, each “new” weed-control tactic may not kill 100 per cent of the weeds. “But they are going to be additive,” Owen said. “You’re going to support the herbicide chemistry that we have.”

“Growers need to understand that an escaped weed here or there is a problem. Scouting is critically important,” Owen said. He hopes public and private programs can create incentives for non-chemical weed management. And, he hopes to change farmers’ mindsets and increase awareness of weed resistance. Ultimately, he wants it to become socially unacceptable for farmers to have resistant weeds. “We need to kind of police our neighbours, to be sure that they aren’t contributing to the problem.”

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